Disturbances in endothelial cell (EC) barrier regulation are a hallmark of lung inflammation, angiogenesis and cancer. EC permeability is regulated by a balance between contractile and tethering forces and critically depends upon the coordinate rearrangement of actin and microtubule cytoskeleton. Growing evidence indicates that inflammatory cytokines like TGF-B increase EC permeability in vitro and are involved in the increase in lung permeability in vivo. TGF-B elicits cellular effects on endothelium by engagement of TGF-B type I receptors, ALK1 and ALK5, following by activation of specific SMAD proteins that control the transcription of target genes. However, the involvement of ALK/SMAD signaling in TGF-B-induced cytoskeletal rearrangement and permeability are virtually unexplored. Our novel preliminary data indicated that specific depletion or inhibition of ALK5 and SMAD4 proteins significantly attenuated TGF-B decrease in transendothelial electrical resistance (TER) indicating the involvement of ALK/SMAD signaling in TGF-B-induced EC barrier compromise. Our recent data also indicate that TGF-B-induced decrease in TER and formation of paracellular gaps is tightly linked to F-actin stress fiber formation and increases in myosin light chain (MLC) phosphorylation indicating the involvement of contractile mechanisms in TGF-B-induced EC permeability. TGF-B-induced changes in EC cytoskeleton are critically dependent upon Rho GTPase activity and microtubule remodeling, but not Ca2+ signaling or MLC kinase activation. cAMP activation attenuates both TGF-B-induced decreases in TER and increases in MLC phosphorylation supporting the involvement of cAMP/PKA in barrier protection against TGF-B-induced EC permeability. In addition, TGF-B-induced EC stimulation activates p38 MAP kinase pathway, which also potentially can be involved in Rho-independent EC contractility via phosphorylation of key cytoskeletal proteins, like caldesmon and HSP-27. However, the link between activation of ALK/SMAD signaling and activation of EC contractility is unknown. In this proposal, we will explore the role of SMAD dependent and independent pathways involved in TGF-B-induced EC barrier dysfunction. Engagement of ALK1 and ALK5 receptors will be temporally linked with regulatory SMAD proteins phosphorylation and activation of Rho- and p38 MAPK-mediated EC cytoskeletal rearrangement and permeability. In Specific Aim 1, we will examine the link between TGF-B-induced Rho activation, ALK/SMAD signaling and EC permeability. In Specific Aim 2, we will examine the link between p38 MAPK-dependent pathways involved in TGF-B-induced EC barrier dysfunction and ALK/SMAD signaling. In Specific Aim 3, we will explore the molecular mechanisms by which cAMP/PKA protects against TGF-B-induced EC barrier failure focusing on the SMADs, Rho, p38 and cytoskeletal proteins as potential PKA targets. These studies will provide an understanding of novel signaling pathways involved in cytokine-mediated lung EC barrier regulation and promise new directions and targets for treatment of lung disorders.